Organic & Biomolecular Chemistry
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2.17–2.24 (m, 1H), 3.45–3.49 (m, 1H), 3.57–3.64 (m, 3H), 5.07–5.10 (d, J = 12 Hz, 1H), 5.13–5.16 (d, J = 12 Hz, 1H),
3.96–4.00 (dd, J = 4.18, 18 Hz, 1H), 4.10–4.15 (dd, J = 4.18, 18 6.79–6.82 (m, 1H), 6.94–6.95 (d, J = 7.6 Hz, 1H), 7.14–7.15 (m,
Hz, 1H), [4.44 (m, 1H)], 4.55–4.58 (m, 1H), 5.12–5.14 (d, J = 3H), 7.22–7.28 (m, 3H), 7.34–7.37 (m, 10H), 7.30 (s, 1H),
12.5 Hz, 1H), 5.19–5.21 (d, J = 12.5 Hz, 1H), 6.51 (s, 1H), 7.51–7.54 (d t, J = 2, 2, 8 Hz, 1H), 8.35–8.36 (d, J = 2 Hz, 1H),
7.28–7.38 (m, 10H). 13C NMR (125 MHz, CDCl3) δ (22.1), 24.5, 8.38–8.39 (dd, J = 1.6, 4.8 Hz, 1H). 13C NMR (100 MHz, CDCl3)
28.9, (31.3), (41.9), 42.1, 43.5, 45.9, (46.6), (58.6), 58.9, 67.0, δ 24.7, 27.3, 33.9, 37.1, 39.9, 42.2, 43.4, 46.6, 47.6, 54.3, 60.1,
(67.5), 127.3, 128.1, 128.3, 128.5, (128.6), (128.7), 128.9, 129.4, 66.5, 123.7, 126.7, 127.3, 128.3, 128.4, 128.5, 128.6, 128.9,
134.5, (134.9), 135.4, 166.9, (167.1), (170.9), 171.0, (171.1), 129.3, 129.4, 133.1, 134.6, 135.6, 137.1, 137.9, 147.6, 150.3,
171.5. HRMS m/z calcd for C22H25N2O4 [M + H]+ 381.1808, 168.8, 169.6, 170.9, 171.3, 171.7. HRMS m/z calcd for
found 381.1802.
C40H44N5O6 [M + H]+ 690.3286, found 690.3287.
Phenylacetyl-(2R)-alaninyl-(2S)-proline benzyl ester (22b). As
Phenylacetyl-(2R)-alaninyl-(2S)-prolyl-(2S)-3-pyridinylalani-
described above for carbamate 20b (1 g, 2.65 mmol), the Boc nyl-(3S)-β-homophenylalanine benzyl ester (24b). 24b was
group was removed with HCl to give white powder. The hydro- prepared using the same hydrogenation and coupling pro-
chloride salt (858 mg, 2.65 mmol) was treated with phenylace- cedures as described above for 24a, employing benzyl ester
tic anhydride (1.35 g, 5.31 mmol) and triethylamine (3.72 mL, 22b (110 mg, 0.28 mmol) and palladium-on-carbon (10 wt%,
26.5 mmol), and the residue after evaporation was purified by 30 mg) to give the acid. Acid 23b (187 mg, 0.61 mmol) was
flash chromatography on silica gel using 7% MeOH in EtOAc coupled to 3-(pyridyl)alaninyl-β-homophenylalaninyl benzyl
to afford ester 22b (916 mg, 78% yield) as white powder: Rf = ester hydrochloride 12 (230 mg, 0.56 mmol) using HOBt
0.45 (10% MeOH in EtOAc); mp 140 °C; [α]2D0 −3.8 (c 1.0, (91 mg, 0.67 mmol), TBTU (216 mg, 0.67 mmol), and DIEA
1
MeOH); H NMR (500 MHz, CDCl3) showed a 3 : 1 mixture of (292 μL, 1.68 mmol). Purification of the residue by flash
prolyl amide isomers δ [1.17 (m, 3H)], 1.29–1.33 (m, 3H), chromatography on silica gel using 8% MeOH in EtOAc
[1.84 (m, 2H)], 1.95–2.03 (m, 2H), 2.14–2.25 (m, 2H), 3.50–3.53 afforded peptide 24b (320 mg, 81% yield) as white powder: Rf
(m, 3H), 3.74–3.77 (m, 1H), 4.49–4.50 (m, 1H), 4.80–4.84 = 0.38 (10% MeOH in EtOAc); [α]2D0 −12.4 (c 1.5, MeOH); mp
(m, 1H), 5.08–5.09 (m, 2H), 6.69–6.73 (m, 1H), [6.88 (m, 1H)], 70 °C; 1H NMR (500 MHz, CDCl3) δ 1.32–1.33 (d, J = 7 Hz, 3H),
[7.27 (m, 10H)], 7.28–7.35 (m, 10H). 13C NMR (125 MHz, 1.37–1.46 (m, 1H), 1.77–1.83 (m, 1H), 1.92–1.97 (m, 2H),
CDCl3) δ (17.6), 18.2, (22.4), 24.6, 29.0, (31.1), (43.0), 43.5, 2.53–2.62 (m, 2H), 2.76–2.81 (m, 1H) 2.85–2.90 (dd, J = 8.5,
(46.6), (46.7), 46.8, 46.9, 59.2, (59.4), 66.7, (67.3), 127.1, 128.0, 13.5 Hz, 1H), 3.00–3.04 (dd, J = 6, 14 Hz, 1H), 3.19–3.23 (dd, J =
128.2, (128.3), 128.5, (128.6), 128.7, 129.2, 134.9, (134.96), 6, 14.5 Hz, 1H) 3.37–3.42 (m, 1H), 3.47–3.50 (d, J = 15.5 Hz,
(135.2), 135.6, 170.1, (170.7), 171.1, 171.5, (172.10), (172.18); 1H), 3.52–3.55 (d, J = 15.5 Hz, 1H), 3.74–3.81 (td, J = 3, 9 Hz,
HRMS m/z calcd for C23H27N2O4 [M + H]+ 395.1965, found 1H), 4.35–4.40 (m, 1H), 4.42–4.44 (m, 1H), 4.48–4.58 (m,
395.1960.
2H), 5.04–5.06 (d, J = 12.5 Hz, 1H), 5.08–5.11 (d, J = 12.5 Hz,
Phenylacetyl-glycinyl-(2S)-prolyl-(2S)-3-pyridinylalaninyl-(3S)- 1H), 6.52–6.53 (d, J = 4.5 Hz, 1H), 7.14–7.28 (m, 12H),
β-homophenylalanine benzyl ester (24a). A solution of phenyl- 7.30–7.37 (m, 5H), 7.43–7.45 (d, J = 8.5 Hz, 1H), 7.58–7.60 (d,
acetyl-glycinyl-(2S)-proline benzyl ester 22a (100 mg, J = 8 Hz, 1H), 8.42–8.43 (d, J = 4.5 Hz, 1H), 8.47 (s, 1H).
0.26 mmol) in anhydrous EtOH (60 mL) and AcOH (6 mL) was 13C NMR (125 MHz, CDCl3) δ 16.1, 24.1, 28.7, 33.1, 37.6, 40.3,
treated with palladium-on-carbon (10 wt%, 30 mg) and stirred 42.6, 47.4, 47.9, 48.2, 54.8, 61.0, 66.4, 123.49, 126.6, 127.4,
under 1 atm of hydrogen overnight. The mixture was filtered 128.1, 128.3, 128.4, 128.5, 128.9, 129.40, 129.42, 134.1, 134.2,
on Celite™, which was washed with hot MeOH, and the com- 135.9, 136.8, 137.8, 147.5, 150.4, 170.2, 171.0, 171.1, 172.3,
bined filtrate and washings were evaporated and freeze-dried 172.4. HRMS m/z calcd for C41H46N5O6 [M + H]+ 704.3442,
to give acid 23a. Phenylacetyl-glycinyl-(2S)-proline (23a, found 704.3446.
172 mg, 0.61 mmol) was dissolved in 12 mL of dichloro-
Phenylacetyl-glycinyl-(2S)-prolyl-(2S)-3-pyridinylalaninyl-(3S)-
methane, treated with HOBt (91 mg, 0.67 mmol) and TBTU β-homophenylalanine (3). 3 was obtained from ester 24a
(216 mg, 0.67 mmol), stirred for 10 min, treated with (100 mg, 0.14 mmol) by hydrogenation with palladium-on-
3-(pyridyl)alaninyl-β-homophenylalaninyl benzyl ester hydro- carbon (10 wt%, 17 mg) as described above. Purification by
chloride (12, 230 mg, 0.56 mmol), followed by drop-wise preparative HPLC on a C18 reverse-phase column gave acid 3
addition of DIEA (292 μL, 1.68 mmol), and stirred overnight. (30 mg, 36% yield) as white powder: 1H NMR (700 MHz,
Evaporation of the volatiles gave a residue, which was purified CD3OD) detected a 4.5 : 1 mixture of prolyl amide isomers δ
by flash chromatography on silica gel using 8% MeOH in [1.62 (m, 2H)], 1.75–1.85 (m, 2H), 1.89–1.94 (m, 1H), 2.05–2.10
EtOAc. Evaporation of the collected fractions afforded peptide (m, 1H), [2.22 (m, 1H)], 2.44 (s, 2H), 2.85–2.86 (d, J = 7 Hz, 2H),
24a (274 mg, 72%) as yellow oil: Rf = 0.4 (10% MeOH in 2.88–2.92 (m, 1H), 3.12–3.15 (m, 1H), 3.53–3.56 (m, 1H),
EtOAc); [α]2D0 −23.5 (c 1.0, MeOH); 1H NMR (400 MHz, CDCl3) δ 3.58–3.70 (m, 3H), 4.04 (s, 2H), 4.36–4.38 (m, 1H), 4.41–4.43
1.79–1.87 (m, 2H), 1.90–1.93 (m, 1H), 2.17–2.19 (m, 1H), (m, 1H), 4.50–4.52 (m, 1H), [4.64 (m, 1H)], 7.20–7.36 (m, 11H),
2.52–2.54 (d, J = 5.6 Hz, 2H), 2.80–2.94 (m, 3H), 3.14–3.19 (dd, 7.69–7.72 (m, 1H), [8.20 (s, 2H)], 8.35–8.39 (m, 2H); 13C NMR
J = 5.2, 14 Hz, 1H), 3.30–3.37 (m, 1H), 3.40–3.45 (m, 1H), 3.67 (175 MHz, CD3OD) δ (21.8), 24.2, 28.8, (31.8), 34.0, (34.7), 37.7,
(s, 2H), 3.76–3.81 (dd, J = 3.6, 17.2 Hz, 1H), 3.99–4.04 (dd, J = 39.7, (41.1), 41.9, 42.10, (42.15), 46.5, (46.8), 48.2, 54.0, (59.7),
5.6, 17.2 Hz, 1H), 4.41–4.44 (m, 1H), 4.47–4.54 (m, 2H), 60.4, 123.7, (126.0), 126.1, 126.5, (126.6), (127.9), 128.0, 128.22,
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem., 2015, 13, 7750–7761 | 7759